Current treatments for asthma, such as glucocorticosteroids, leukotriene receptor antagonists and mast cell stabilisers, focus on alleviating symptoms and improving lung function through modulation of immune cell function. Despite the use of these drugs, bronchial hyperresponsiveness continues to deteriorate as airway remodelling persists (1). Previous findings revealed that the phytocannabinoid Δ9-tetrahydrocannabinol (THC) is a bronchodilator (2) with anti-inflammatory action (3). The aim of this study was to investigate the effects of THC on bronchial epithelial cell permeability after exposure to pro-inflammatory TNFα. Calu-3 human bronchial epithelial cells were grown for 21 to 28 days in 12-transwell plates at air-liquid interface to allow development of tight junctions. Changes in epithelial permeability were measured using transepithelial electrical resistance (TEER) at various time points. Cells were incubated with THC (30 µM) for one hour, then exposed to TNFα (10 ng/mL) over 48 hours. In a similar experiment set up, the potential role of cannabinoid (CB) receptors were identified with an hour pre-incubation of CB1 antagonist AM251 (100 nM) or CB2 antagonist SR144528 (1 µM) prior to adding THC (30 µM) and TNFα (10 ng/mL). The effect of repeated THC administration on the development of transepithelial resistance was evaluated by measuring TEER over 10 days after exposure to THC (3, 10 or 30 µM). Cell expression of cannabinoid receptors was determined by Western blotting. Treatment with THC (30 µM) prevented TNFα-induced reduction in TEER which persisted for 48 hours (%TEER with TNFα alone = 62.4±13.0%, t = 4 hours, compared to 86.2±3.8% in the presence of THC n=5, p<0.01 two-way ANOVA). This effect of THC was itself attenuated by either AM251 (100 nM; p<0.05) or SR144528 (1 µM; p<0.001). In the absence of TNFα, THC produced a concentration-dependent increase in transepithelial resistance over 10 days (%TEER with 30 µM THC = 161.7%±11.9% of control at t = 10 days, compared to 128.7%±2.4% with 10 µM and = 120±3.6% with 3 µM (n=3, p<0.01, two-way ANOVA compared to vehicle control (0.3% ethanol)). CB1 and CB2 receptor-like immunoreactivity was found in Calu-3 cells. These findings demonstrate that THC reverses the reduction in transepithelial resistance caused by TNFα, through an effect at CB1 and CB2 receptors. Repeated treatment with THC results in a time-dependent increase in epithelial resistance, indicative of reduced epithelial permeability. These data indicate that THC, or perhaps other cannabinoid receptor ligands, could be beneficial in the prevention of inflammation-induced changes in airway epithelial cell permeability which are an important feature of airways diseases.